To reduce exhaust emissions, vehicles are fitted with one or more emission control devices, such as catalytic converters. These devices frequently utilize rapid heating to a light-off temperature in order to function optimally. Various approaches have been to taken to rapidly heat the emission control devices. In one example, the engine may be operated at a rich air-fuel ratio. Uncombusted hydrocarbons remaining in the exhaust may react with secondary air injected into the exhaust manifold to produce additional heat in the exhaust upstream of the emission control devices.
European Patent No. EP2016261 discloses a secondary air injection pump that injects air into an exhaust port. However, in a multi-cylinder, four-stroke engine, the exhaust is pulsed and hence the exhaust ports do not release exhaust gases at the same time. Thus, the exhaust ports releasing exhaust gases have a higher backpressure than the exhaust ports not releasing exhaust gas. During secondary air injection, this may result in the exhaust ports releasing exhaust gases receiving less secondary air than the exhaust ports not releasing exhaust gases.
The inventors have recognized the issues with the above approach and offer a system to at least partly address them. In one embodiment, a multi-cylinder engine method comprises combusting a rich air-fuel mixture, injecting air into a first cylinder's exhaust port and not into a second cylinder's exhaust port during the first cylinder's exhaust stroke, and injecting air into the second cylinder's exhaust port and not into the first cylinder's exhaust port during the second cylinder's exhaust stroke.
In this way, each exhaust port may receive an injection of secondary air (or other oxidizing agents) only when that port is releasing exhaust gas. The amount of secondary air needed for efficient reaction with the hydrocarbons in the exhaust may be provided equally for all ports, eliminating unnecessary air injection.
In one example, the injected air may be provided via an air pump that includes a rotating disc mechanically timed to the engine's camshaft. By only injecting air into each port during that port's exhaust release, a smaller air pump or air source may be used that utilizes less energy, thus improving overall engine fuel economy. Further, as the rotating disc of the air pump may be coupled to the camshaft to control the air injection to each port, the control strategy of the air pump and injection into the ports may be simplified. Also, the use of thermally resistant control valves at each port may be eliminated, reducing costs, if desired.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.